When drilling or completing wells in earth formations, various fluids are used in the well for a variety of reasons. Common uses for well fluids include: lubrication and cooling of drill bit cutting surfaces while drilling generally or drilling-in (i.e., drilling in a targeted petroliferous formation), transportation of “cuttings” (pieces of formation dislodged by the cutting action of the teeth on a drill bit) to the surface, controlling formation fluid pressure to prevent blowouts, maintaining well stability, suspending solids in the well, minimizing fluid loss into and stabilizing the formation through which the well is being drilled, fracturing the formation in the vicinity of the well, displacing the fluid within the well with another fluid, cleaning the well, testing the well, transmitting hydraulic horsepower to the drill bit, placing a packer, abandoning the well or preparing the well for abandonment, and otherwise treating the well or the formation.
Fluids or muds typically include a base fluid and weighting agents to help remove cuttings and other solids from the well. During drilling and wellbore treatments, the fluid is injected through the center of the drill string to the bit and exits through the annulus between the drill string and the wellbore. During this process, the fluid may cool and lubricate the bit and/or transport drill cuttings and other solids to the surface. At the surface, a portion of the drill cuttings can be separated from the fluid and the fluid can be circulated back into the well for reuse.
Drill cuttings can originate from different geological strata, including clay, rock, limestone, sand, shale, underground salt mines, brine, water tables, and other formations while other solids may include metal shards from tools and downhole equipment. These solids can range in size from less than two microns to several hundred microns. Drill cuttings are commonly classified according to size: smaller than 2 microns are classified as clay; from 2 to 74 microns, silt; 74 to 500 microns, sand; and larger than 500 microns, cuttings. Several types of separation devices have been developed to efficiently separate the varied sizes of the weighting materials, drill cuttings, and solids from the drilling fluid, including shakers (shale, rig, screen), screen separators, centrifuges, hydrocyclones, desilters, desanders, mud cleaners, mud conditioners, dryers, filtration units, settling beds, sand traps, and the like.
A typical process used for the separation of drill cuttings and other solids from the fluid may include multiple steps which separate solids from fluids according to the size of the solids. Fluid returned from a well and containing drill cuttings, additives, and other solids can be fed to a shale shaker. The shale shaker may separate the fluid into large particles, such as drill cuttings, and effluent. The fluid and remaining particles within the effluent can then be passed through a degasser, a desander to remove sand, a desilter to remove silt, and a centrifuge to remove smaller particles. The solids, including any weighting materials, are then discarded and the resulting clean fluid can be reused. In some cases, solids may pass through the degasser, desander, desilter, and centrifuge, thus ending up with the clean fluid.
In some separating systems, the clean fluid can retain a portion of solids which can be recycled downhole with the clean fluid. In some cases the solids can damage the formation and/or downhole equipment. Additionally, small solids can accumulate in the downhole fluid which may also be detrimental to the formation. Further, if the solids content increases, additional drilling or completions fluid (water, oil, etc.) may be added to dilute the fluid and to maintain the desired weight of the fluid. The dilution of the fluid containing solids is often costly and can alter the balance of chemical and fluid proportions.
Separating systems having separating devices such as, for example, shakers, screen separators, centrifuges, and hydrocyclones, may also be used to separate solids from drilling fluids that are commonly used to treat and maintain wellbores. Some separating systems may allow a portion of solid particles to pass through the separating devices along with the desired clean fluid, and the solids can also be recycled into the wellbore with the clean fluid. As described above, solid particles may damage formation and/or downhole equipment.
Accordingly, there exists a need for a system and method for effectively separating solids from fluids to obtain a fluid having low solids and/or low gravity solids content.